Quick FMU question
#1
Burning Brakes
Thread Starter
Quick FMU question
When supercharging, does the FMU have to be installed after the normal fuel pressure regulator or can it be installed before it? I see most people install the FMU last, before the fuel returns to the tank. I have future plans of running a low boost supercharger and a wet 50 or 75 shot of nitrous. If the FMU could be installed before the normal fuel pressure regulator I could pull the fuel for the nitrous kit from after the FMU and before the regulator. If not, I'll need a separate regulator and return line just for the nitrous kit. Is there any reason the FMU is normally installed after the normal regulator?
#2
Drifting
Member Since: Dec 2001
Location: providence ut
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the fmu increases fuel pressure by restricting the return line size inside of the fmu thus increasing the fuel pressure...........it must be in the return line........just pull fuel from the fuel rail, thats what I did.
#3
Drifting
You could swap places and put the stock reglator after the FMU. Then you will get 'stock' fuel pressure between the FMU and stock regulator.
They must both be mounted in series on the fuel return after the fuel rail. The one that wants the highest pressure wins.
They must both be mounted in series on the fuel return after the fuel rail. The one that wants the highest pressure wins.
#4
Burning Brakes
Thread Starter
But when the FMU raises the fuel pressure substantially, doesn't that screw with the nitrous enrichment fuel? You could jet it accordingly but as the boost pressure goes up it would screw up the air fuel ratio, I think.
#5
Burning Brakes
Thread Starter
That is what I was hoping for and thinking of trying. It seems right, was just hoping someone who has tried it could confirm. Have you mounted an FMU after the fuel rail but before the afpr and had it work ok? I was thinking the same as you, the one with the higher pressure "wins".
#6
Drifting
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forced induction is not easy..............your playing on the edge using both blower and nos..........even with a low boost and low nos shot....I would recomend one or the other...............maybe a small dry nos shot just to cool the supercharger charge.
#7
Burning Brakes
Thread Starter
Anyway, what difference does it make if I spray a wet or dry shot? A wet shot will cool the intake charge just the same. Just have to spray fuel in also, like I do now. I have a Zex wet kit on it now. Do I have to spray a dry shot?
Using the nitrous to cool the charge is what is so impressive on a supercharged engine. I am planning on running an intercooler and also running 1 degree of timing retard per pound of boost (with a timing retard box). Should be safe there, will datalog to make sure. With a little more timing retard and a 40 or 50 shot it should make some serious power. If I keep air fuel ratio and the timing good and safe it may even survive for a year on the stock bottom end.
Last edited by dan0617; 08-11-2008 at 11:32 AM.
#8
Drifting
Both the FMU and a stock regulator works exactly the same. They increase the fuel pressure by restricting the return flow. They just have a different pressure ratio. The one that wants to restrict the flow most will set the pressure.
What I said before is not true however. I thought about this again.
If the FMU wants 50 psi and the stock regulator 30, then the FMU will win and both see a reference pressure of 50 psi. As the stock regulator sees 50 psi it will fully open. The pressure differance over the stock regulator will be almost nothing and almost all 50 psi pressure drop will be over the FMU.
What I said before is not true however. I thought about this again.
If the FMU wants 50 psi and the stock regulator 30, then the FMU will win and both see a reference pressure of 50 psi. As the stock regulator sees 50 psi it will fully open. The pressure differance over the stock regulator will be almost nothing and almost all 50 psi pressure drop will be over the FMU.
#9
Burning Brakes
Thread Starter
Both the FMU and a stock regulator works exactly the same. They increase the fuel pressure by restricting the return flow. They just have a different pressure ratio. The one that wants to restrict the flow most will set the pressure.
What I said before is not true however. I thought about this again.
If the FMU wants 50 psi and the stock regulator 30, then the FMU will win and both see a reference pressure of 50 psi. As the stock regulator sees 50 psi it will fully open. The pressure differance over the stock regulator will be almost nothing and almost all 50 psi pressure drop will be over the FMU.
What I said before is not true however. I thought about this again.
If the FMU wants 50 psi and the stock regulator 30, then the FMU will win and both see a reference pressure of 50 psi. As the stock regulator sees 50 psi it will fully open. The pressure differance over the stock regulator will be almost nothing and almost all 50 psi pressure drop will be over the FMU.
Also, I'm assuming that an FMU can't be set to 45 psi when vacuum or no boost is present, otherwise you wouldn't need a "stock" regulator at all, you could just set the FMU to 45 psi. I'm assuming that the FMU just takes whatever pressure it is seeing and begins raising it on a ratio when boost starts. Is that correct? I don't know much about FMU's and am learning here.
#10
Safety Car
There has been a lot of discussion of which regulator will take precedence (FMU or AFPR). The one that requests the higher psi (which will be the FMU when WOT under boost) will take precedence no matter whether it is mounted upstream or downstream of the factory regulator.
However, if you mounted the FMU first, then fed the N2O from the feed between the FMU and the latter mounted factory AFPR, then you would have a lower psi feed for the N2O system. But, I would be concerned about getting a consistant feed of fuel to the N2O system with an FMU that is "hi-jacking" the fuel away from the N2O feed at rates of 50-85 psi in mere fractions of a second. I would not go this route.
I ran a seperate system (actually most of the factory system) to feed just the N2O system in my car. I used a modded factory TPI regulator and fed off one side to a -6AN to feed the fuel solenoid and returned from the modded factory regulator through the original return line. The modded AFPR has the vacuum line capped for a consistant psi at all times. The engine fuel supply is a completely separate system feeding from the Weldon pump through an aftermarket fuel system.
Aaron
However, if you mounted the FMU first, then fed the N2O from the feed between the FMU and the latter mounted factory AFPR, then you would have a lower psi feed for the N2O system. But, I would be concerned about getting a consistant feed of fuel to the N2O system with an FMU that is "hi-jacking" the fuel away from the N2O feed at rates of 50-85 psi in mere fractions of a second. I would not go this route.
I ran a seperate system (actually most of the factory system) to feed just the N2O system in my car. I used a modded factory TPI regulator and fed off one side to a -6AN to feed the fuel solenoid and returned from the modded factory regulator through the original return line. The modded AFPR has the vacuum line capped for a consistant psi at all times. The engine fuel supply is a completely separate system feeding from the Weldon pump through an aftermarket fuel system.
Aaron
#11
Burning Brakes
Thread Starter
Thank you for the answers! I was worried about the nitrous feed not getting any fuel for an instant or 2, I was considering not spraying till after the boost was in quite a bit till I knew the FMU was returning some fuel and not holding it all back. If it won't work then I'd be better to get bigger injectors and a new tune then to run a whole separate fuel system with the way my car is now.
Another possible option, is there a type of regulator or reducer that lets a certain psi through no more, no return line needed? Maybe I could pull nitrous fuel from the fuel rail, then use this device in the feed line to the nitrous fuel solenoid. That way, when the FMU raises the fuel pressure, this device would only let a set psi (say 45 psi) through it. Anyone ever heard of such a device?
Another possible option, is there a type of regulator or reducer that lets a certain psi through no more, no return line needed? Maybe I could pull nitrous fuel from the fuel rail, then use this device in the feed line to the nitrous fuel solenoid. That way, when the FMU raises the fuel pressure, this device would only let a set psi (say 45 psi) through it. Anyone ever heard of such a device?
#12
Drifting
The pressure between the FMU and the "stock" regulator would be whatever the "stock" regulator is set at, which would be about 45 psi.
Also, I'm assuming that an FMU can't be set to 45 psi when vacuum or no boost is present, otherwise you wouldn't need a "stock" regulator at all, you could just set the FMU to 45 psi. I'm assuming that the FMU just takes whatever pressure it is seeing and begins raising it on a ratio when boost starts. Is that correct? I don't know much about FMU's and am learning here.
Also, I'm assuming that an FMU can't be set to 45 psi when vacuum or no boost is present, otherwise you wouldn't need a "stock" regulator at all, you could just set the FMU to 45 psi. I'm assuming that the FMU just takes whatever pressure it is seeing and begins raising it on a ratio when boost starts. Is that correct? I don't know much about FMU's and am learning here.
No ... Read the post where I corrected myself again.
When the FMU is in control the fuel pressure before the stock regulator could drop to nothing when mounted after the FMU.
If the FMU is a 8:1 unit and it is set to 45 psi at atmospheric reference pressure and the stock regulator 1:1 is set to 45 psi, this is what you get:
Code:
manifold regulator FMU psi psi psi -5 40 5 stock regilator wins -1 44 37 stock regilator wins 0 45 45 switchover point 1 46 53 FMU wins 5 50 85 FMU wins 10 55 125 FMU wins